The mouse mammary tumor virus (MMTV) induces primarily breast cancers in mice. However, some highly related MMTV strains (e.g., TBLV) induce T-cell tumors, but not mammary carcinomas. The major differences between (LTRs) at the ends of pro-viral DNA. These differences include (i) loss of negative regulatory elements (NREs) that suppress MMTV transcription in lymphoid tissues, including thymus, (ii) loss of the C-terminal one-third of the superantigen gene that is involved in MMTV transmission from lymphoid to mammary cells, and (iii) triplication of 62 bp flanking the LTR deletion. In this grant application, regions of the TBLV genome that are responsible for thymotropism will be assessed. First, the specific cell types infected by TBLV will be analyzed, and optimal routes of infection will be determined. Second, specific mutations in known NREs, the triplicated region of the sag open reading frame will be introduced into a mammotropic MMTV and tested for their ability to infect specific cell types and to cause leukemias. Third, the triplicated region shown to enhance MMTV transcription in T-cells will be analyzed for the binding of cellular transcription factors, and these factors will be identified. Fourth, if the LTR is not sufficient to reproduce the cell-type tropism and leukemogenicity of TBLV, we will construct chimeras between mammotropic and thymotropic strains of MMTV and test these chimeras for their ability to infect different cell types and induce disease. In the second specific aim, we will continue our studies to assess the role of specific TBLV integration sites in virally-induced leukemias. Previously identified TBLV integrations will be analyzed by PCR and by pulsed field gel electrophoresis. Integrations in the c-myc locus also will be assessed in leukemias induced by MMTV pro-viruses that have a truncated sag gene and lack the NREs, but also lack the specific triplicated region found in TBLV. These experiments and studies of transgenic mice that have a mutant MMTV or TBLV LTR upstream of c-myc should provide information about whether LTR changes affect integration site choice or the ability to stimulate c-myc expression. These results should elucidate the factors conducive for the development of human leukemia and permit the development of novel strategies for their treatment.